The primary metering rod is tapered, getting smaller towards the tip. While hard to see, the rod should have a number stamped on it. The number will correspond to the last 2 numbers in our number nomenclature, i.e. 130-39, might have a stamping of 39B. The 39 indicates the larger diameter of the rod is .039".
The small diameter end is the power end. When the engine is idling, or at low power, vacuum is at the highest and the bigger diameter is in the main metering jet. As you accelerate, the vacuum drops, raising the rod so that the smaller end is in the jet, allowing more fuel to pass.